1. Field of the Invenrion
This invention is concerned with a method for disposing of industrial wastes or, more specifically, petroleum sludge in a delayed coking process for heavy petroleum fractions.
2. The Prior Art
Delayed coking has been practiced in the petroleum industry for many years for the purpose of extracting a maximum amount of liquid products from reduced petroleum crudes or heavy residua.
In the delayed coking process, a reduced crude oil or residual petroleum fraction is heated to coking temperatures and is fed into a large holding vessel or coke drum under conditions which promote thermal cracking and polymerization to produce light hydrocarbon distillate fractions which pass overhead to a fractionator, and solid petroleum coke which remains in and eventually fills the drum.
In the usual practice of the delayed coking process, a residual oil from a fractionator wherein the lighter products have been separated by distillation, the resulting residual oil is pumped through a furnace where it is heated to the required coking temperature and then discharged into the bottom of a coke drum. The heated residual oil enters the coke drum at a temperature from about 875.degree. to 950.degree. F. The contents of the coke drum are held at these thermal cracking temperatures during the period it is being charged.
After a predetermined filling time, the contents in the coke drum are cooled down in a series of distinct steps. First stage cooling is effected by passing steam into the coke drum for a sufficient period of time to cool the contents of the drum down to about 675.degree. to 725.degree. F. This steam cooling period also serves as a means in which any remaining volatile hydrocarbons are steam distilled or "stripped" from the coke bed and which are first recovered from the overhead of the coke drum by the coker fractionator and then later by a blowdown or vapor recovery system. Cooling in a second stage begins with the introduction of water into the coke drum. In this second stage of cooling, the water is converted to steam which serves to further promote the removal of additional vaporizable hydrocarbons produced in the coke drum. In the final cooling stage, liquid water cools the coke to a temperature usually less than 212.degree. F. which will permit it's mechanical removal from the coke drum making the unit available for a fresh charge of residual oil for coking.
An important aspect of the delayed coking process is the quality of the coke that is produced. The coke is a marketable product. High quality coke meeting certain specifications as to its volatile content can be marketed at a premium price as green coke suitable for calcining to anode quality carbon for use in the electrolytic processing industries. On the other hand, coke that does not meet these specifications is only useful as fuel coke and has a lower economic value.
With the advent of strict environmental laws with respect to the disposal of industrial wastes, such as petroleum sludge, the delayed coking process which produces a large body of high temperature coke maintained under thermal cracking conditions has been proposed for decomposing or destructively reducing treatable industrial wastes thereby effectively disposing of them.
U.S. Pat. No. 4,666,585 discloses a method for the disposal of petroleum sludge wherein petroleum sludge is added to the hot liquid hydrocarbon feedstock or charge as it is being fed into the coke drum at the start of the coking process.
U.S. Pat. No. 3,917,564 discloses a method for the disposal of industrial and sanitary wastes wherein liquid sludge containing dispersed combustible matter and fine discrete solid particles is added to liquid water in an intermediate cooling step in the coke cooling cycle.
The foregoing methods for disposing of industrial wastes or petroleum sludge may have significant limitations with respect to the amount of waste material that can be disposed of in a single delayed coking cycle when in anode quality green coke production, as with U.S. Pat. No. 3,917,564, and by possible limiting delayed coking unit charge rate, as with U.S. Pat. No. 4,666,585.